Don’t expect to see the planet Neptune tonight (September 7, 2014), even though tonight’s waxing gibbous moon shines fairly close to this world on the sky’s dome. Even on the darkest of moonless nights, you need an optical aid to see Neptune, the eighth planet outward from the sun. Neptune now reigns as the farthest full-fledged planet in the solar system because Pluto was reclassified as a “dwarf planet” by the International Astronomical Union (IAU) in 2006.
But, although you won’t see Neptune tonight, you can use tonight’s moon to get a feel for the whereabouts of Neptune and the zodiacal constellation Aquarius in the starry heavens. Aquarius lies to the north of the bright star Fomalhaut, which will be visible tonight – despite the moonlit glare.
Because the ecliptic – the roadway of the sun, moon and planets – passes right through the constellation Aquarius, practiced sky gazers know any planet must be on or near the ecliptic. And right they are, for Neptune will reside close to the 5th-magnitude zodiacal star Sigma Aquarii for the rest of this year. This star, Sigma Aquarii, is visible to the unaided eye as a faint speck of light in a dark country sky.
Once the moon leaves the evening sky by mid-September, try locating Neptune in front of the constellation Aquarius, and near the star Sigma Aquarii. Be forewarned. You’ll need an optical aid, and probably a good sky chart plus lots of patience to see this faint and distant planet.
Sky chart showing the star Sigma Aquarii
Pluto, by the way, is locked into a 2:3 orbital resonance with Neptune. For every two times that Pluto circles the sun, Neptune does so three times. Beyond Neptune, there are a number of trans-Neptunian objects – called plutinos – that are locked into a 2:3 orbital resonance with Neptune.
Pluto wasn’t denied planetary status because it’s not massive enough to be a planet. Believe it or not, Pluto – in and of itself – has sufficient mass to qualify. Rather, Pluto was reclassified by the IAU because this world doesn’t have the heft to “clear the neighborhood around its orbit.”
Bottom line: Tonight – September 7, 2014 – the moon passes fairly close to Neptune on our sky’s dome. In 2006, Neptune replaced Pluto as the solar system’s most distant planet.